Influence of tannic acid in Leacril/Rhodamine B system: Thermodynamics aspects

In this work, we present data of the thermodynamics and kinetics analysis of the dyeing process of Leacril with a cationic dye, Rhodamine B, and the effect that the pretreatment of the fibre with a polar surfactant, tannic acid, has in these factors. We observed that the adsorption of cationic dyes onto the fibre is favoured by the temperature of the system. But, if the fibre has been previously treated with the polar surfactant, the temperature disfavour this adsorption. The experimental time-sorption isotherms are represented by the exponential kinetic equation, Mt = Meq(1 − e−kt), where Mt and Meq refer to the amounts of dye uptake Leacril at time t and equilibrium, respectively, and k refers to the rate constant. We observe that the empirical rate constant increase with temperature in the adsorption process of dye onto untreated Leacril, while this constant decrease if the adsorption takes places onto Leacril pretreated with tannic acid. Besides, this constant decrease its value with the treatment of the fibre that means that the process is slower when the fibre has tannic acid preadsorbed as it can been appreciated on the values of half-adsorption time. Changes of enthalpy and entropy related to the adsorption process in the two cases, Leacril untreated and Leacril pretreated with tannic acid are calculated from the Clausius-Clapeyron equation. Isosterics heat of adsorption are positives when the fibre is untreated, taking values between 8 and 42.5 kJ/mol, but if the fibre present tannic acid preadsorbed these values are negatives and those are between −85 and −5 kJ/mol. These data shows that the process, takes place by different interactions. In our opinion, this fact indicates an increased of the hydrophobic interactions, probably, hydrogen bonding between phenolic hydroxy groups of the acid and the carboxyl group of dye, besides electrostatic interaction between sulfonate and sulphate end groups of Leacril and the cation of the Rhodamine B. Finally, the electrokinetic behaviour of the system has been studied. We can see that the presence of tannic acid onto the fibre give rise to an increased of the absolute value of zeta potential. This increased would be due to the presence of more molecules of Rhodamine B onto the fibre with its carboxyl group ionised as to the molecules of tannic acid.